Process for the transformation annealing of steels



United States The invention relates to a process for the transformationannealing of hypereutectoid and ledeburitic steels, irrespective ofwhether they contain alloy elements in addition to carbon, that is tosay, irrespective of whether they are to be regarded as alloyed orunalloyed. These steels include, for example, high-speed steels,low-alloy and high-alloy tool steels, structural steels of many variedtypes and stainless and acid-resisting steels, in so far as these arehardenable, that is to say, steels containing chromium and only arelatively small proportion of nickel, molybdenum or the like.

The aforesaid types of steels are present in various forms for thepurposes of transformation annealing. They may be in the form of rods,tubes or sheets, and it is furthermore possible that they may have to besubjected to the treatment in the form of wound wire rings. Moreover,they may exist in the form of forgings. By far the commonest form isthat of the rod or the tube.

The heat treatment of these relatively varied steels has hitherto had tobe carried out in a series of different processes. The consequence ofthis is that in a precious steel works the individual products must bedivided into a large number of groups for separate treatment, the numberof the said groups depending upon the composition of the steel to betreated, and the form in which it is present, that is to say, uponwhether it is in the form of sheets, tubes or rods, while regard mustalso be had to the dimensions and the nature of the furnaces availablefor this work. As is well known, the annealing has hitherto generallybeen carried out in car type bottom annealing furnaces or in semi-muffleor muffle furnaces, in more or less large stacks. Each stack passesthrough the annealing stages of heating it, maintaining it at therequired temperature and cooling it. The furnace must participate in allthe temperature variations and it has therefore been proposed to employfurnaces of the type which allows the stack to be successively fed tozones heated at different temperatures. Processes in which material isannealed in stacks are attended by the known disadvantages that uniformtemperature conditions do not exist throughout the stack, because theoutside of the stack reaches higher temperatures than the interior andvice versa. Above all, some of the parts lag considerably in thevariations of temperature. In order to obviate these ditficulties, ithas already been proposed to feed the individual workpieces continuouslyor step-by-step through the individual heating zones of appropriatelyconstructed furnaces.

While the method of continuous and progressive heating, in particular ofindividual workpieces, affords advantages both with regard to qualityand with regard to the time factor as compared with pure annealing instacks, the heating times are nevertheless still relatively long and inaddition this method is generally practicable only when there are largequantities of similar material to be annealed, which is not generallythe case. Generally, the materials differ greatly from one another andthe groups of material require individual treatment.

I have found that it is possible by applying a preliminary treatmentwhich is identical for all the aforesaid alloyed and unalloyed steels,not only to effect a substan tial shortening of the heating times, butalso to effect the atent G ice annealing of all the aforesaid groups ofsteels with only a small difference in their after-treatment. Theprocess according to the invention resides in that the material to beannealed is first heated to temperatures between 800 C. and 880 C.Preferably, a temperature of about 840 C. is commonly applied to allsteels whether they be hypereutectoid or ledeburitic, this temperaturebeing maintained for one to two hours. After this treatment, thematerial o-t be annealed is cooled at any desired rate to a temperaturelower than 700 C. preferably to a temperature of about 650 C. Loweringof the temperature to less than 600 C. in this stage only results in thedisadvantage of heat losses.

As soon as the material under treatment has reached this temperature, itis heated to the actual annealing temperature. This annealingtemperature must be maintained for one to three hours.

The annealing temperature which must actually be maintained depends uponthe composition of the material concerned. A particular advantage of theprocess according to the invention has been found to reside in that twodifferent temperature levels are generally sufiicient in this stage ofthe treatment.

Unalloyed and low-alloyed hypereutectoid steels are heated in this stageat 7 40 C.-760 C., preferably at 750 C. For more highly alloyedhypereutectoid steels and for ledebunitic steels, a temperature of 780C.-'0 C. must be maintained. In special cases, however, for example inthe case of highly alloyed high-speed steels or highly alloyed toolsteel containing about 2% of carbon and 12% of chromium, a temperatureof up to 850 C. is expedient'ly applied in this stage of the treatment.

If the aforesaid heating times and heating temperatures, applied to theappropriate steel qualities, are observed, this type of treatment issufficient in all cases to produce the desired structure formation andannealing hardness.

This treatment is followed by the cooling, which is uniform for alltypes of steels, and is effected at any desired rapidity to about 720 C.and then slowly down through the temperature range from 720 C. to 680 C.Cooling may be in accordance with a constant temperature curve or mayproceed in temperature steps, for example by maintaining the material atsuccessive stages of 720 C., 700 C. or the like. Regardless of themanner in which the temperature is controlled during the cooling, thematerial must be cooled from about 720 C. to about 680 C. in about twoto four hours.

It will be apparent that the total times of treatment are thussubstantially reduced as compared with those of all the known processes,and that in general only two different treatment temperatures arenecessary in the actual annealing stage for the purpose of adaption tothe different qualities of steel.

The process according to the invention may be carried out in aparticularly advantageous manner by progressively passing the steelmaterial to be annealed through an annealing'furnace, the temperature ofwhich can be regulated in zones. In such a furnace, the individual zonescan be adjusted to and maintained at exactly predetermined temperatures.The material to be annealed is passed through this furnace at such aspeed that the heating-up, maintenance and cooling times are positivelyobtained. No disadvantage occurs if the material to be annealed, forexample a rod, is excessively long in relation to such a temperaturezone of the furnace. The result of the heat treatment is not affected ifthe steel material, or unit of material, to be annealed is movedconstantly forwards and lies simultaneously in two or more treatmentzones.

The advantages afforded by the process according to the invention willhereinafter be explained with reference to a number of examples.

It will be assumed that it is desired to subject a hypereutectoid rollerbearing steel containing about 1% of carbon, 1.5 of chromium and theusual proportions of silicon, manganese and the usual impurities to atransformation annealing.

In the known stack annealing, tubes or rods of this material are treatedtogether in stack units, usually of about tons. The annealing timesamount on an average to about 48 hours. The whole stack is heated up forabout 20 hours, to reach a temperature of 800 C. This temperature ismaintained for 10 hours, whereafter cooling to 720 C. is effected inabout 4 hours and this temperature is maintained for 8 to 10 hours. 6hours must thereafter be expended in reaching a temperature of about 600C. to 650 C. before the stack can be withdrawn from the furnace.

The necessary time could be reduced to about 20 hours by passing thematerial continuously through the furnace.

If, on the other hand, the process according to the invention isadopted, the total treatment time is reduced to 8 to 12 hours. For thishypereutectoid steel, an annealing temperature of 780 C.800 C. must bemaintained for about 3 hours.

A steel having the same chromium content, but containing 1.5 of carboncan be treated in a similar manner. A steel, the composition of whichlies at the boundary between that of a hypereutectoid steel and that ofa ledeburitic steel and which contains 0.4% of carbon and 13% ofchromium, may be treated in a similar manner. -Ledeburitic steels, suchfor example as a high-speed steel containing 18% of tungsten, 4% ofchromium, and 1.5% of vanadium or high-speed steel having a low contentof materials in short supply and containing 3% of molybdenum, 3% oftungsten, 3% of vanadium and 4% of chromium are also capable of beingtreated in the same manner, as also is a ledeburitic steel containing 4%of chromium, 1% of molybdenum, 4% of vanadium and 12% of tungsten. Ifthe usual processes are applied, annealing times of 45 hours and moreare necessary with these steels, a particular temperature in the 760C.860 C. range having to be maintained for each composition, that is tosay, there will be a large number or different groups to be subjected tothe annealing.

It will be seen from these examples that the process is applicable tosteels of very different compositions, and even unalloyed hypereutectoidsteels, for example a steel containing 1% of carbon and the usual minorproportions of manganese and silicon, can be treated in this manner ifheated to and maintained at an actual annealing temperature of 740C.-760 C., preferably 750 C.

The process according to the invention, particularly when applied tohypereutectoid steels, affords the advantageous result that the carbidesare given a particularly fine and uniform grain size. This is due to thefirst heating and cooling treatment, which removes the unfavourableinfluences of the antecedents of the material, and to the shortannealing times. In addition, it is possible to vary the actual carbidegrain size by suitable choice of the annealing temperature within theaforesaid limits.

In the case of high-speed turning steels, the additional advantage isobtained that the danger of decarburization is substantially avoidedowing to the short times of treatment.

The process according to the invention is not comparable to a heattreatment fluctuating around the upper transformation point, which isusual with hypereutectoid steels for the recrystallisation, primarily ofthe basic mass, that is to say, of the ferrite grain. On the other hand,the process has the object of transforming the carbides, thistransformation being initiated .by a heating to temperatures between 800C. and 880 C. prior to the actual annealing.

What I claim is:

1. A process for the transformation annealing of a steel selected fromthe group consisting of hypereutectoid and ledeburitic steels, byprogressively passing the steel through a furnace the temperature ofwhich is zonally regulated, which comprises the steps of (1) heating thesteel to a temperature between 800 C. and 880 C. and maintaining thesteel at that temperature for a period of from the order of from 1 to 2hours; (2) cooling the steel at any desired rate to a temperature below700 C. but not substantially below 600 C.; (3) immediately reheating thesteel to an annealing temperature between 740 C. and 800 C. andmaintaining the steel at said annealing temperature for a period of theorder of from 1 to 3 hours, and wherein said annealing temperature isselected from the lower part of said temperature range for the unalloyedand low alloyed hypereutectoid steels and in the upper part of saidtemperature range for high alloyed steels; (4) cooling the steel at anydesired rate to a temperature about 720 C. and thereafter cooling saidsteel through the temperature range of from 720 C. to 680 C. during aperiod of time of the order of from 2 to 4 hours.

2. The process of claim 1, wherein said steel is an unalloyedhypereutectoid steel and said annealing temperature is between 740 C.and 760 C.

3. The process of claim 1, wherein said steel is a lower alloyedhypereutectoid steel, and said annealing temperature is between 740 C.and 760 C.

4. The process of claim 1, wherein said steel is a high alloyed steeland said annealing temperature is between 780 C. and 800 C.

5. Process for the transformation annealing of a high ly alloyedhigh-speed steel according to claim 1 in which the re-heating is to anannealing temperature of up to 850 C.

6. Process for the transformation annealing of a highly alloyed toolsteel according to claim 1 in which the re-heating is to an annealingtemperature of up to 850 C.

7. A process for the transformation annealing of a steel selected fromthe group consisting of hypereutectoid and ledeburitic steels, byprogressively passing the steel through a furnace the temperatrue ofwhich is zonally regulated, which comprises the steps of (1) heating thesteel to a temperature between 800 C. and 880 C. and maintaining saidtemperature for a period of from the order of 1 to 2 hours; (2) coolingthe steel at any desired rate to a temperature lower than 70 C. but notsubstantially lower than 600 C.; (3) immediately re-heating said steelto an annealing temperature between 740 C. and 800 C. and maintainingthe steel at said annealing temperature for a period of the order offrom 1 to 3 hours, the annealing temperature being selected from thelower part of said range for the unalloyed and low alloyedhypereutectoid steels and in the upper part of said temperature rangefor highly alloyed steels; (4) cooling the steel to a temperature ofabout 680- C., said cooling being carried out in two steps, the firststep comprising cooling the steel to 740 C. at a rate faster than therate of cooling in the second stage, said second stage comprisingcooling the steel through the temperature range from 720 C. to 680 C. ata rate between 10 and 20 per hour.

8. A process for the simultaneous transformation annealing of aplurality of steels selected from one of the groups consisting of (a)unalloyed and low-alloyed hypereutectoid steels and (b) highly alloyedhypereutectoid and ledeburitic steels, by progressively passing saidsteels through a furnace the temperature of which is zonally regulated,which comprises the steps of (1) heating said steels to a temperaturebetween 800 C. and 880 C. and maintaining said steels at thattemperature for a period of from the order of from 1 to 2 hours; (2)cooling said steels at any desired rate to a temperature below 700 C.but not substantially below 600 C.; (3)

immediately reheating said steels to an annealing temperature between740 C. and 800 C., and maintaining said steel at said annealingtemperature for a period of the order of from 1 to 3 hours, and whereinsaid annealing temperature is selecetd in the lower part of saidtemperature range for the unalloyed and low alloyed hypereutectoidsteels and in the upper part of said temperature range for high alloyedsteels; (4) cooling said steels at any desired rate to a temperatureabout 720 C.; and thereafter cooling said steels through the temperaturerange of from 720 C. to 680 C. during a period of time of the order offrom 2 to 4 hours.

9. A process for the transformation annealing of a plurality of steelsselected from the group consisting of hypereutectoid and ledeburiticsteels, by progressively passing said steels through a furnace thetemperature of which is zonally regulated, which comprises the steps of(1) heating said steels to a temperature between 800 C. and 880 C. andmaintaining said temperature for a period of time of from the order of 1to 2 hours; (2) 20 cooling said steels at any desired rate to atemperature lower than 700 C. but not substantially lower than 600 C.;(3) immediately reheating said steels to an annealing temperaturebetween 740 C. and 800 C. and

maintaining said steels at said annealing temperature for 25 a period ofthe order of from 1 to 3 hours, the annealing temperature being selectedfrom the lower part of said range for the unalloyed and low alloyedhypereutectoid steels and in the upper part of said temperature rangefor highly alloyed steels; (4) cooling said steels to a temperature ofabout 680 C., said cooling being carried out in two steps, the firststep comprising cooling said steels to 720 C. at a rate faster than therate of cooling in the second stage, said second stage comprisingcooling said steels through the temperature range of from 720 C. to 680C. at a rate between 10 and 20 per hour.

References Cited in the file of this patent UNITED STATES PATENTS BonteDec. 31, 1946 Seabold Feb. 12, 19 52 OTHER REFERENCES

1. A PROCESS FOR THE TRANSFORMATION ANNEALING OF A STEEL SELECTED FROMTHE GROUP CONSISTING OF HYPEREUTECTOID AND LEDEBURITIC STEELS, BYPROGRESSIVELY PASSING THE STEEL THROUGH A FURNACE THE TEMPERATURE OFWHICH IS ZONALLY REGULATED, WHICH COMPRISES THE STEPS OF (1) HEATING THESTEEL TO A TEMPERATURE BETWEEN 800*C. AND 880*C. AND MAINTAINING THESTEEL AT THAT TEMPERATURE FOR A PERIOD OF FROM THE ORDER OF FROM 1 TO 2HOURS, (2) COOLING THE STEEL AT ANY DESIRED RATE TO A TEMPERATURE BELOW700*C. BUT NOT SUBSTANTIALLY BELOW 600*C. (3) IMMEDIATELY REHEATING THESTEEL TO AN ANNEALING TEMPERATURE BETWEEN 740*C. AND 800*C. ANDMAINTAINING THE STEEL AT SAID ANNEALING TEMPERATURE FOR A PERIOD OF THEORDER OF FROM 1 TO 3 HOURS, AND WHEREIN SAID ANNEALING TEMPERATURE ISSELECTED FROM THE LOWER PART OF SAID TEMPERATURE RANGE FOR THE UNALLOYEDAND LOW ALLOYED HYPEREUTECTOID STEELS AND IN THE UPPER PART OF SAIDTEMPERATURE RANGE FOR HIGH ALLOYED STEELS, (4) COOLING THE STEEL AT ANYDESIRED RATE TO A TEMPERATURE ABOUT 720*C, AND THEREAFTER COOLING SAIDSTEEL THROUGH THE TEMPERATURE RANGE OF FROM 720*C. TO 680*C. DURING APERIOD OF TIME OF THE ORDER OF FROM 2 TO 4 HOURS.